The present invention relates to mobile communication network systems and their transmission technology migration. In particular, the invention relates to migration to IP (Internet Protocol) technology of the A-interface.
In mobile communication networks, e.g. networks according to 3GPP (Third Generation Partnership Project) Release 4 architecture, a core network, e.g. an MSC (Mobile Switching Centre) server system, is used to connect different access networks, e.g. BSS (Base Station Subsystem) to each other. In Release 4 networks, MSC functionality is split into two logical functions, which typically are provided by physically separate network elements, namely a Media Gateway (MGW) and an MSC server (MSS). Control functions of the MSC are provided by the MSS. Bearer switching functions of the MSC are provided by the MGW.
In mobile communication networks, many domains and interfaces between these domains have been adapted to IP technology. Today's general trend is to use IP transmission as widely as possible. BSS over IP is a technique trend in wireless network evolution, which can construct high bandwidth, high efficiency and low cost basic networks. BSS over IP involves Gb-interface and A-interface over IP. Gb-interface over IP has been standardised in 3GPP Release 4, and for A-interface over IP, control plane signalling over IP (SIGTRAN) has been introduced in 3GPP Release 8.
Traditionally, the user plane of the A-interface is based on TDM (Time Division Multiplexing) transmission technology. In order to take full advantage of IP based technologies, protocols of A-interface user plane should be adapted for IP based transport. One of the main advantages of having IP based A-interface also for the user plane is a much more flexible network design between the BSS and the CS (Circuit Switched) core network. The A-interface over IP (AoIP) can also simplify implementation of Multipoint A feature in a pooled network. Furthermore, UTRAN (UMTS (Universal Mobile Telecommunications System) Terrestrial Radio Access Network) and more advanced RAN (Radio Access Network) can use a common IP backhaul with GERAN (GSM (Global System for Mobile communication) Edge (Enhanced Data rates for Global Evolution) Radio Access Network).
Based on 3GPP AoIP work item, a GSM (Global System for Mobile communication) BSS communicates with the core network over the A-interface (A reference point) which is an Mb like interface (IP interface, “Mb access (GSM)”).
In Release 8, 3GPP has standardized the user plane of A-interface over IP (AoIP) as an alternative for legacy A-interface over TDM (AoTDM). The standard supports two options:                AoIP with TC transcoding (TC) in BSS        AoIP with TC in MGW        
The second option “AoIP with TC in MGW” seems to be an option which will be most widely implemented. Main reasons for this are the benefits that “AoIP with TC in MGW” provides: No separate TC elements are needed in the BSS network. Common transcoding resources located in the core network/MGW can be used for 2G (second generation mobile communications network), 3G (third generation mobile communications network), and SIP/IMS (Session Initiation Protocol/Internet Multimedia Subsystem) traffic. IP bandwidth savings can be reached by transmitting compressed mobile codec in the AoIP interface instead of G.711 Pulse Code Modulation (PCM) speech codec. MGW call handling capacity savings and optimal speech quality are achieved by 2G TrFO (Transcoder Free Operation). Cost savings are reached due to the reduced number of network elements because no separate TC is needed in BSS network for AoIP calls. Network implementation and maintenance is easier because of the reduced number of network elements and amount of TDM transport.
However, there is a large amount of existing GSM networks with TDM based A-interface. When AoIP with TC in MGW feature is introduced in a network, there are no means to utilize the existing 2G transcoders, such as Transcoder and Rate Adaptation Unit (TRAU), and TDM lines in a practical way without affecting on speech quality or causing extra load (Digital Signal Processing) in MGW.